1. Field of the Invention
The present invention relates to a tilt device for a marine propulsion unit.
2. Description of the Related Art
There is a structure described in Japanese Patent No. 2945071 as a tilt device with a gas assist function for assisting a tilt up load applied to a main body of a propulsion unit by a gas pressure in a pressure accumulating chamber.
The tilt device is structured such that a piston is fixed to the piston rod inserted to the cylinder. The inner portion of the cylinder is separated into a rod side chamber and a piston side chamber by the piston. A switching valve for communicating the rod side chamber with the piston side chamber and an operating portion thereof are provided in a side portion of the cylinder. A pressure accumulating chamber for applying a gas pressure for a gas assist to the piston side chamber is arranged within the cylinder and on the piston side chamber. Accordingly, the gas pressure in the pressure accumulating chamber is applied to both sides of the piston via a working fluid when opening the switching valve, whereby the gas assist force corresponding to a cross sectional area of the rod can be generated in a tilting up direction.
Further, an absorber valve can be opened when the fluid pressure in the rod side chamber is increased to a level equal to or more than a predetermined value in order to absorb an impact force applied to the propulsion unit main body. When a collision with drifting wood occurs, the fluid in the rod side chamber can be discharged and a return valve can return the temporarily discharged fluid mentioned above from the absorber valve to the rod side chamber. This series of events will return the propulsion unit main body after rising up. The absorber valve, rod side chamber and return valve are provided in the piston. Further, a free piston for coinciding the return position of the propulsion unit main body after rising up with a staying position before rising up is provided in the piston close to the piston side chamber, whereby it is possible to form a liquid discharging chamber for the fluid discharged from the absorber valve between the piston and the free piston.
(1) Since the absorber valve and the return valve are provided in the piston, it is hard to make the diameter of the piston small and further, it is unavoidable that the cylinder is enlarged.
(2) Since the pressure accumulating chamber is within the single cylinder type cylinder and on the piston side chamber, in a recessed portion in a side of the closed end of the cylinder, it is hard to increase a capacity of the pressure accumulating chamber.
(3) Since the switching valve and the operating portion thereof are provided in the side portion of the cylinder, the lateral size of the tilt device becomes too large.
An object of the present invention is to provide a tilt device for a marine propulsion unit with a gas assist function which can increase the capacity of the pressure accumulating chamber while being compact and can secure a rising up performance of a propulsion unit main body caused by a collision of drifting wood.
In accordance with the present invention, the tilt device for a marine propulsion unit contains a cylinder connected to one of a hull and a main body of a propulsion unit and receiving a working fluid, and a piston rod connected to another and inserted into the cylinder so as to freely compress and expand. A piston connected to an inserted end to the cylinder of the piston rod and defines a rod side chamber in the receiving side of a piston rod and a piston side chamber in a non-receiving side of the piston rod within the cylinder. A switching valve which is capable of communicating the rod side chamber with the piston side chamber, and a pressure accumulating chamber capable of applying a gas pressure to the piston side chamber are contained in the marine propulsion unit. The cylinder is constituted by a double pipe which has an outer cylinder and an inner cylinder, and receives the piston rod and the piston in the inner cylinder. The outer cylinder and the inner cylinder are sealed at an end portion of the cylinder, a rod guide supporting the piston rod is provided, a free piston is arranged in a space between the outer cylinder and the inner cylinder, the pressure accumulating chamber is formed in an opposite rod guide side to the free piston in said space, and a liquid discharging chamber is formed in a rod guide side. An absorber valve which can be opened when the fluid pressure in the rod side chamber is increased to a level equal to or more than a predetermined value can feed the fluid in the rod side chamber to the liquid discharging chamber. A return valve which can be opened when the pressure of the fluid in the liquid discharging chamber is increased to a level equal to or more than a predetermined value can feed the fluid in the liquid discharging chamber to the rod side chamber are provided in the rod guide.